Semiconductor device and fabricating method thereof

ABSTRACT

A semiconductor device and a method of fabricating the semiconductor device are provided. The semiconductor device includes a substrate, a gate structure over the substrate, a source/drain regions adjacent to the pair of spacers in the substrate, an etch stop layer next to the gate structure and overlying the substrate, a contact plug extending into the source/drain region and partially overlapping the gate structure, a protective layer over the etch stop layer overlying the substrate and covering the etch stop layer next to the gate structure without the contact plug, and an interlayer dielectric layer over the protective layer. The contact plug has no contact-to-gate short issue to the gate structure.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/961,638, filed Dec. 7, 2015, which is a divisional of U.S. patentapplication Ser. No. 14/191,282, filed Feb. 26, 2014, now U.S. Pat. No.9,231,067, the disclosures of which are incorporated herein by referencein their entirety.

BACKGROUND

The semiconductor industry has experienced rapid growth. The fabricationof an integrated circuit (IC) is focusing on the increase the number ofthe ICs with the miniaturization of the respective ICs in a wafer. An ICdevice includes various microelectronic components, such asmetal-oxide-semiconductor field effect transistors (MOSFETs). Further,an MOSFET include several components, such as a gate electrode, gatedielectric layer, spacers, and diffusion regions of source and drainregions. Typically, an interlayer dielectric (ILD) layer is deposited tocover the MOSFETs, followed with the electrical connections by formingthe contact plugs in the ILD layers connecting the source/drain regions.With the size shrinkage of the IC devices, both of the gate length andthe distance between the MOSFETs decrease, which may result in variousissues such as contact shorting in the fabrication of the IC device.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a cross-sectional view of the semiconductor device accordingto various embodiments of the present disclosure;

FIGS. 2A-2I are cross-sectional views at various stages of fabricatingthe semiconductor device according to various embodiments of the presentdisclosure; and

FIG. 3 is a cross-section& view of the semiconductor device according tovarious embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

As used herein, the terms “comprising,” “including,” “having,”“containing,” “involving,” and the like are to be understood to beopen-ended, i.e., to mean including but not limited to.

The singular forms “a,” “an” and “the” used herein include pluralreferents unless the context clearly dictates otherwise. Therefore,reference to, for example, a dielectric layer includes embodimentshaving two or more such dielectric layers, unless the context clearlyindicates otherwise. Reference throughout this specification to “oneembodiment” or “an embodiment” means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure.Therefore, the appearances of the phrases “in one embodiment” or “in anembodiment” in various places throughout this specification are notnecessarily all referring to the same embodiment. Further, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments. It should be appreciatedthat the following figures are not drawn to scale; rather, these figuresare intended for illustration.

As the semiconductor device pitch decreases, the structure of thecontact plug also needs to adjust. According to various embodiments ofthe present disclosure, a way to design the contact plug is forming thecontact plug directly next to the gate structure to save the spacebetween the contact plug and the other gate structure. According tovarious embodiments of the present disclosure, another way to design thecontact plug is extending a part of the contact plug, which is above thegate structure, to make the other contacts aligned with the contact plugeasily. In forming this part-extension contact plug, an isolation layeris deposited over the gate structure to prevent contact to gate shortissue. However, according to various embodiments of the presentdisclosure, when combining the above mentioned two methods for thecontact plug structure, another issue of contact to gate short hasbrought out. The formation of the opening for the contact plug, which isdirectly next to the gate structure, includes exposing part of thesource/drain region and part of the isolation layer, the isolation layermay be overetched during the etching process to cause the contact togate short. Therefore, a mechanism of forming a semiconductor device isprovided.

Referring to FIG. 1, FIG. 1 is a cross-sectional view of thesemiconductor device according to various embodiments of the presentdisclosure. The semiconductor device 100 includes a substrate 110, agate structure 120 over the substrate 200, a source/drain regions 130adjacent to the gate structure 120 in the substrate 110, an etch stoplayer 140 next to the gate structure 120 and overlying the substrate110, a contact plug 170 extending into the source/drain region 130 andpartially overlapping the gate structure 120, a protective layer 150over the etch stop layer 140 overlying the substrate and covering theetch stop layer 140 next to the gate structure 120 without the contactplug 170, and an interlayer dielectric layer 160 over the protectivelayer 150. The gate structure 120 includes a gate dielectric layer 122over the substrate 110, a gate electrode 124 over the gate dielectriclayer 122, an isolation layer 126 over the gate electrode 124, and apair of spacers 128 next to the two sides of the gate electrode 124.

In various embodiments of the present disclosure, the substrate 110 mayinclude silicon. The source/drain regions 130 may dope boron,phosphorous, or arsenic. The gate dielectric layer 122 may includesilicon oxide, and the gate electrode 124 may include polysilicon. Invarious embodiments of the present disclosure, the gate dielectric layer122 may include high-k dielectric material such as HfO₂, HfSiO, HfSiON,HfTaO, HfTiO, or HfZrO, and the gate electrode 124 may include metalsuch as aluminum, copper, tungsten, or metal alloys. The isolation layer126 may include silicon nitride (SiN). A thickness of the isolationlayer 126 is in a range from about 5 nm to about 50 nm. The spacers 128may include silicon nitride. The etch stop layer 140 may include siliconnitride (Si₃N₄). A thickness of the etch stop layer 140 is in a rangefrom about 1 nm to about 15 nm. The protective layer 150 may includeoxide with refractive index in a range from about 1.4 to about 2, suchas silicon oxynitride (SiON). And a thickness of the protective layer150 is in a range from about 1 nm to about 5 nm. The interlayerdielectric layer 160 may include a material with the refractive index ina range from about 2.5 to about 4, such as flowable oxide. And thecontact plug 280 may include tungsten.

According to various embodiments of the present disclosure, theprotective layer 150 is used to protect the substrate. When thethickness of the etch stop layer 140 is close to the thickness of theisolation layer 126, the isolation layer 126 may be penetrated duringthe etching operation to break through the etch stop layer 140 to formthe contact plug 170 in connection with the source/drain region 130.Therefore, according to some embodiments, the semiconductor device 100may have contact to gate short issue due to the low etching selectivelybetween the etch stop layer 140 and the isolation layer 126. A way tosolve the above mentioned problem is to decrease the thickness of theetch stop layer 140. But in case the thickness of the etch stop layer140 decreases, the oxygen in the operation of annealing the interlayerdielectric layer 160 may penetrate through the etch stop layer 140 andreach the source/drain region 130, which results in higher contactresistance and may even induce the oxidization of the substrate. Theprotective layer 150 including high quality oxide, which includes oxidewith refractive index in a range from about 1.4 to about 2, such assilicon oxynitride (SiON), deposited over the etch stop layer 140 mayprevent the oxygen penetration issue. Also the protective layer 150 hashigh etching selectivity with the etch stop layer 140 and the isolationlayer 126, so the isolation layer 126 may not be etched when breakingthrough the protective layer 150. The thickness of the etch stop layer140 therefore may be decreased to avoid the isolation layer 126 to bepenetrated during the etching operation to break through the etch stoplayer 140 to form the contact plug 170 in connection with thesource/drain region 130.

Referring to FIGS. 2A-2I, FIGS. 2A-2I are cross-sectional views atvarious stages of fabricating the semiconductor device according tovarious embodiments of the present disclosure. Referring to FIG. 2A, agate structure 216 includes a dummy gate electrode 212 and a pair ofspacers 214 are formed over a substrate 200. A source/drain regions 210are adjacent to the gate structure 216 in the substrate 200. Thesubstrate 200 includes a semiconductor material like silicon, germanium,carbon, another semiconductor material as an III-V or II-VI material, orcombinations thereof. In embodiments, the substrate 210 comprises acrystalline silicon substrate (e.g., wafer). Further, the substrate 210may include an epitaxial layer, which is strained, and/or asilicon-on-insulator (SOI) structure. According to various embodimentsof the present disclosure, the substrate 200 may include active regionsincluding various doping configurations, such as p-wells and n-wells,depend on the design requirements. The source/drain regions 210 may bedoped with p-type or n-type dopants. For example, the source/drainregions may be doped with p-type dopants, such as boron or BF2; n-typedopants, such as phosphorus or arsenic; and/or combinations thereof.

The gate electrode 212 may include polysilicon, and the spacers 214 mayinclude a dielectric material such as silicon nitride, silicon oxide,silicon carbide, silicon oxynitride, other suitable materials, and/orcombinations thereof. In some embodiments, the spacers 214 may include amultilayer structure. The gate structure 216 may be formed by anysuitable process. For example, the gate structure 216 may be formed bydeposition, photolithography patterning, and etching processes, and/orcombinations thereof. The deposition processes may include chemicalvapor deposition (CVD), physical vapor deposition (PVD), atomic layerdeposition (ALD), plasma enhanced CVD (PECVD), high density plasma CVD(HDPCVD), metal organic CVD (MOCVD), remote plasma CVD (RPCVD),epitaxial growth methods (e.g., selective epitaxy growth), sputtering,plating, spin-on coating, other suitable methods, and/or combinationsthereof. The photolithography patterning processes may includephotoresist coating (e.g., spin-on coating), soft baking, mask aligning,exposure, post-exposure baking, developing the photoresist, rinsing,drying (e.g., hard baking), other suitable processes, and/orcombinations thereof. The etching processes may include dry etching, wetetching, and/or other etching methods (e.g., reactive ion etching). Theetching process may also be either purely chemical (plasma etching),purely physical (ion milling), and/or combinations thereof.

Referring to FIG. 2B, according to various embodiments of the presentdisclosure, an etch stop layer 220 is deposited over the gate structure216 and the substrate 200. The etch stop layer 220 may include siliconnitride (Si₃N₄). The etch stop layer 220 may be deposited by anysuitable methods such as CVD. The thickness of the etch stop layer 220may be in a range from about 1 nm to about 15 nm.

Referring to FIG. 2C, according to various embodiments of the presentdisclosure, a protective layer 230 is deposited over the etch stop layer220. The protective layer 230 may include high quality oxide, in whichthe high quality means the refractive index of the oxide is in a rangefrom about 1.4 to about 2. For example, the protective layer 230 mayinclude SiO₂, SiOCN, SiON, and/or combinations thereof. The protectivelayer 230 may be deposited by any suitable methods such as high densityplasma CVD (HDP-CVD) or ALD. The protective layer 230 may prevent theoxygen diffusion through the etch stop layer 220 into the substrate 200.Therefore the thickness of the etch stop layer 220 may decrease toprevent the contact to gate issue. The thickness of the protective layer230 may be in a range from about 1 nm to about 5 nm.

Referring to FIG. 2D, according to various embodiments of the presentdisclosure, an interlayer dielectric layer 240 is deposited over theprotective layer 230. Because the semiconductor device pitch is scalingdown, for example, the space between two metal gates is less than 50 nm,the material of the interlayer dielectric layer 240 needs good gap-fillcapability to fill the space. The good gap-fill capability material maybe an oxide with a refractive index in a range from about 2.5 to about4. In various embodiments of the present disclosure, the material of theinterlayer dielectric layer 240 may be a flowable oxide, and theinterlayer dielectric layer 240 may be disposed by flowable CVD (FCVD)or other suitable deposition method.

Referring to FIG. 2E, according to various embodiments of the presentdisclosure, the semiconductor device is planerized to expose the topsurface of the gate structure 216. The part of interlayer dielectriclayer 240, the protective layer 230, and the etch stop layer 220 abovethe top surface of the gate structure 216 are removed by chemicalmechanical polishing (CMP) to expose the gate structure 216. Thesemiconductor device is than annealed with oxygen to densify theinterlayer dielectric layer 240. The protective layer 230 which includeshigh quality oxide may barrier the oxygen diffusion in the annealprocess, therefore may protect the substrate not to he oxidized.

Referring to FIG. 2F, according to various embodiments of the presentdisclosure, the dummy gate electrode 212 is removed and replaced by ametal gate electrode 254. For example, in various embodiments, the dummygate electrode 212 is replaced by a high-k dielectric layer 250 over thesubstrate between the two spacers 214, a work function layer 252 overthe high-k dielectric layer 250, and a metal gate electrode 254 over thework function layer 252 to form a high-k metal gate (HKMG) structure.The dummy gate electrode 212 may be removed to form a trench between thespacers 214 by any suitable process. The high-k dielectric layer 250,work function layer 252, and metal gate electrode 254 may than be formedin the trench of the gate structure 216. The high-k dielectric layer 250may include HfO₂, HfSiO, HfSiON, HfTaO, HfTiO, HfZrO, metal oxides,metal nitrides, metal silicates, transition metal-oxides, transitionmetal-nitrides, transition metal-silicates, oxynitrides of metals, metalaluminates, zirconium silicate, zirconium aluminate, silicon oxide,silicon nitride, silicon oxynitride, zirconium oxide, titanium oxide,aluminum oxide, hafnium dioxide-alumina (HfO₂—Al₂O₃) alloy, othersuitable high-k dielectric materials, and/or combinations thereof. Thework function layer 252 may include TiN, WN, or W for PMOS and TiAl,TiAlN, or TaCN for NMOS or other suitable material having proper workfunction. The metal gate electrode 254 may include a conductivematerial, such as aluminum, copper, tungsten, titanium, tantulum,titanium nitride, tantalum nitride, nickel silicide, cobalt silicide,TaC, TaSiN, TaCN, TiAl, TiAlN, other suitable materials, and/orcombinations thereof. In some embodiments, the high-k dielectric layer250, work function layer 252, and metal gate electrode 254 may includemultiple layers in the gate structure 216. And the high-k dielectriclayer 250, work function layer 252, and metal gate electrode 254 may beformed by any suitable process to any suitable thickness.

Referring to FIG. 2G, according to various embodiments of the presentdisclosure, part of the metal gate electrode 254 is removed and anisolation layer 260 is deposited over the metal gate electrode 254between the two spacers 214. Part of the metal gate electrode 254, thework function layer 252, and the high-k dielectric layer 250 areremoved. Than the isolation layer 260 may be deposited over the metalgate electrode 254 between the two spacers 214 in a self-aligned manner.The isolation layer 260 may protect the metal gate electrode 254 not tocontact with contact plugs. In various embodiments, the isolation layer260 may include silicon nitride (SiN) or other suitable materials. Thethickness of the isolation layer 260 may be thicker than the etch stoplayer 220, so that the isolation layer 260 may not be penetrated whenforming the contact opening. In various embodiments of the presentdisclosure, the thickness of the isolation layer 260 is in a range fromabout 5 nm to about 50 nm. The isolation layer 260 may be formed by anysuitable process such as CVD disclosed herein.

Referring to FIG. 2H, according to various embodiments of the presentdisclosure, a contact opening 270 is formed in the interlayer dielectriclayer 240 and contacts the isolation layer 260 and the source/drainregion 210. The contact opening 270 may be formed next to the gatestructure 216 and over part of the gate structure 216, extend into partof the isolation layer 260 and spacers 214. The contact opening 270 maybe formed by two etching operations. The first etching operation mayetch. part of the interlayer dielectric layer 240 and the protectivelayer 230 next to the spacers 214, than stop at the etch stop layer 220and the isolation layer 260. The first etching process may have etchingselectivity to the etch stop layer 220 and protective layer 230, alsothe isolation layer 260 and the interlayer dielectric layer 240. Becausethe protective layer 230 and the interlayer dielectric layer 240 bothinclude oxide, and the etch stop layer 220 and the isolation layer 260both include nitride. The protective layer 230 therefore may not affectthe etching process, may be etched together with the interlayerdielectric layer 240, and may also protect the substrate not to beoxidized. The thickness of the etch stop layer 220 therefore maydecrease and have no oxygen diffuse through in the annealing process.The second etching process may break through part of the etch stop layer220 to contact the source/drain region. In the mean time, becauseetching selectivity between the isolation layer 260 and the etch stoplayer 220 is low in comparison with the selectivity between siliconnitride and silicon oxide. Part of the isolation layer 260 may also beetched by the second etching process. Because the thickness of theisolation layer 260 is thicker than the etch stop layer 220, theisolation layer 260 may not be overetched so as not to expose the metalgate electrode 254, which avoids the contact-to-gate short issue.

Referring to FIG. 2I, according to various embodiments of the presentdisclosure, a contact plug 280 is formed in. the contact opening 270.The contact plug 280 may be formed in the interlayer dielectric layer240 contacting the isolation layer 260 and extending through theprotective layer 230 and the etch stop layer 220 to contact thesource/drain region 210. The contact plug 280 may partially overlappingthe gate structure 216 through the spacer 214, and extending into theisolation layer 260. The contact plug 280 formed directly next to thegate structure 216 may save the space between the gate structures in thesemiconductor device. And the part of the contact plug 280 overlappingthe gate structure 216 may extend the top surface of the contact plug280, enlarging process window of the further operations.

Referring to FIGS. 2A-2I, according to various embodiments of thepresent disclosure, a method of fabricating the semiconductor isprovided. The fabricating method includes forming a gate structure witha dummy gate electrode and a source/drain regions adjacent to the dummygate structure over a substrate. An etch stop layer is formed over thesubstrate, and a protective layer is deposited over the etch stop layer.Further, an interlayer dielectric layer is deposited over the etch stoplayer. Afterwards, the interlayer dielectric layer is polished andannealed. A metal gate structure is formed by replacing part of thedummy gate structure, and an isolation layer is deposited over the metalgate structure. A contact opening is formed through the interlayerdielectric layer to the source/drain regions and the isolation layer,and a contact plug is formed in the contact opening. According to someembodiments, the operation of forming a metal gate structure byreplacing part of the dummy gate structure includes removing the dummygate electrode, forming a gate dielectric layer in the gate structure;forming a work function layer over the gate dielectric layer, andforming a metal electrode over the work function layer. According tosome embodiments, the operation of forming a contact opening through theinterlayer dielectric layer to the source/drain regions and theisolation layer includes etching part of the interlayer dielectric layerand the protective layer next to the spacers, and etching part of theetch stop layer to contact the source/drain region.

Referring to FIGS. 3, 2C and 2I, FIG. 3 is a cross-sectional view of thesemiconductor device according to various embodiments of the presentdisclosure. The difference between the FIGS. 2I and 3 is that theprotective layer 230 in FIG. 3 is only formed over the etch stop layer220 overlying the substrate 200, but the protective layer 230 in FIG. 2Iis formed over the etch stop layer 220 overlying the substrate 200 andcovering the etch stop layer 220 next to the spacer 214 without thecontact plug 280. The semiconductor device in FIG. 3 may be formed inthe process almost the same with the semiconductor device in FIG. 2I,only when in the operation in FIG. 2C, the protective layer 230 is onlyformed over the etch stop layer 220 overlying the substrate 200, withoutcovering the gate structure 216. The protective layer 230 may be formedby deposition, photolithography patterning, and etching process, and/orcombinations thereof. In some embodiments, the protective layer 230 maybe formed by PVD or other deposition method with poor sidewall stepcoverage, followed by an isotropic etching, such as wet etching.

According to various embodiments of the present disclosure, themechanism of fabricating the semiconductor device is provided. Thedisclosed semiconductor device may broaden the top surface of thecontact plug by forming the contact plug partially overlapping the gatestructure but without contact-to-gate short issue. The protective layerformed over the etch stop layer overlying the substrate may protect thesubstrate not be oxidized by oxygen in the annealing operation,therefore the thickness of the etch stop layer may be eliminated toavoid the isolation layer be over etched in the contact openingformation operation forming the contact-to-gate short issue.

In various embodiments of the present disclosure, the semiconductordevice includes a substrate; a gate structure over the substrateincludes a gate dielectric layer over the substrate; a gate electrodeover the gate dielectric layer; an isolation layer over the gateelectrode; and a pair of spacers next to the two sides of the gateelectrode; a source/drain regions adjacent to the pair of spacers in thesubstrate; an etch stop layer next to the pair of spacers and overlyingthe substrate; a contact plug extending into the source/drain region andpartially overlapping the gate structure through the spacer; aprotective layer over the etch stop layer overlying the substrate andcovering the etch stop layer next to the spacer without the contactplug; and an interlayer dielectric layer over the protective layer.

In various embodiments of the present disclosure, the semiconductordevice includes a substrate; a gate structure over the substrateincludes a gate dielectric layer over the substrate; a gate electrodeover the gate dielectric layer; an isolation layer over the gateelectrode; and a pair of spacers next to the two sides of the gateelectrode; a source/drain regions adjacent to the pair of spacers in thesubstrate; an etch stop layer next to the pair of spacers and overlyingthe substrate; a contact plug extending into the source/drain region andpartially overlapping the gate structure through the spacer; aprotective layer over the etch stop layer overlying the substrate; andan interlayer dielectric layer over the protective layer.

In various embodiments of the present disclosure, the method offabricating semiconductor device includes the following operations. Agate structure with a dummy gate electrode and a source/drain regionsadjacent to the dummy gate structure are formed over a substrate. Anetch stop layer is deposited over the substrate. A protective layer isdeposited over the etch stop layer. An interlayer dielectric layer isdeposited over the etch stop layer. The interlayer dielectric layer ispolished and annealed. A metal gate structure is formed by replacingpart of the dummy gate structure. An isolation layer is deposited overthe metal gate structure. A contact opening is formed through theinterlayer dielectric layer to the source/drain regions and theisolation layer. Further, a contact plug is formed in the contactopening.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method of fabricating semiconductor devicecomprising: forming a dummy gate structure with a dummy gate electrodeand source/drain regions adjacent to the dummy gate structure over asubstrate; depositing an etch stop layer over the source/drain regions;depositing a protective layer over the etch stop layer; depositing aninterlayer dielectric layer over the etch stop layer; annealing theinterlayer dielectric layer; forming a metal gate structure by replacingpart of the dummy gate structure; depositing an isolation layer over themetal gate structure; forming a contact opening through the interlayerdielectric layer to expose the source/drain regions; and forming acontact plug in the contact opening and on the metal gate structure. 2.The method of claim 1, wherein forming the metal gate structure byreplacing part of the dummy gate structure comprises: removing the dummygate electrode; depositing a gate dielectric layer in the gatestructure; depositing a work function layer over the gate dielectriclayer; and forming a metal electrode over the work function layer. 3.The method of claim 1, wherein forming the contact opening through theinterlayer dielectric layer to expose the source/drain regionscomprises: etching part of the interlayer dielectric layer and theprotective layer; and etching part of the etch stop layer to contact thesource/drain regions.
 4. The method of claim 1, wherein in depositingthe protective layer over the etch stop layer, a thickness of theprotective layer is in a range from about 1 nm to about 5 nm.
 5. Themethod of claim 1, wherein in depositing the interlayer dielectric layerover the etch stop layer, a material of the interlayer dielectric layerincludes flowable oxide.
 6. The method of claim 1, wherein in depositingan etch stop layer over the source/drain regions, the etch stop layeroverlies the dummy gate structure and the source/drain regions.
 7. Themethod of claim 6, further comprising polishing the etching stop layerto expose the dummy gate structure after annealing the interlayerdielectric layer.
 8. The method of claim 6, wherein in depositing theprotective layer over the etch stop layer, the protective layerconformally overlies the etch stop layer.
 9. The method of claim 6,wherein in depositing the protective layer over the etch stop layer, theprotective layer overlies the etch stop layer without covering the dummygate structure.
 10. A method of fabricating semiconductor devicecomprising: forming a dummy gate structure with a dummy gate electrodeand source/drain. ns adjacent to the dummy gate structure over asubstrate; depositing an etch stop layer over the dummy gate structureand the source/drain regions; depositing a protective layer over theetch stop layer overlying the source/drain regions; depositing aninterlayer dielectric layer over the protective layer; annealing theinterlayer dielectric layer; polishing the etch stop layer to expose thedummy gate structure; forming a metal gate electrode by replacing partof the dummy gate structure; depositing an isolation layer over themetal gate electrode; forming a contact opening through the interlayerdielectric layer to expose the source/drain regions; and forming acontact plug in the contact opening and on the isolation layer.
 11. Themethod of claim 10, wherein in depositing a protective layer over theetch stop layer overlying the source/drain regions, the protective layerconformally overlies the etch stop layer.
 12. The method of claim 10,wherein forming the metal gate structure by replacing part of the dummygate structure comprises: removing the dummy gate electrode; depositinga gate dielectric layer in the gate structure; depositing a workfunction layer over the gate dielectric layer; and forming a metalelectrode over the work function layer.
 13. The method of claim 10,wherein forming the contact opening through the interlayer dielectriclayer to expose the source/drain regions comprises: etching part of theinterlayer dielectric layer and the protective layer; and etching partof the etch stop layer to contact the source/drain regions.
 14. Themethod of claim 10, wherein in forming the dummy gate structure with thedummy gate electrode and the source/drain regions, the dummy gatestructure further comprises a spacer next to the dummy gate electrode.15. A semiconductor device, comprising: a substrate; a gate structureover the substrate comprising: a gate dielectric layer over thesubstrate; a gate electrode over the gate dielectric layer; an isolationlayer over the gate electrode; and a spacer next to each of the twosides of the gate electrode; source/drain regions adjacent to the pairof spacers in the substrate; an etch stop layer next to the pair ofspacers and overlying the substrate; a contact plug extending into thesource/drain region and partially overlapping the isolation layer; aprotective layer over the etch stop layer overlying the substrate; andan interlayer dielectric layer over the protective layer.
 16. Thesemiconductor device of claim 15, wherein the protective layer coversthe etch stop layer next to the spacer without the contact plug.
 17. Thesemiconductor device of claim 15, wherein a refractive index of amaterial of the interlayer dielectric layer is in a range from about 2.5to about
 4. 18. The semiconductor device of claim 15, wherein arefractive index of a material of the protective layer is in a rangefrom about 1.4 to about
 2. 19. The semiconductor device of claim 15,wherein a material of the protective layer is selected from the groupconsisting of SiON, SiOCN, and SiO₂.
 20. The semiconductor device ofclaim 15, wherein a thickness of the etch stop layer is thinner thanthat of the isolation layer.